Any Chemical Engineers Can Advise on Soda Ash?

We have a soda ash mixing plant that we want to use to make up batches of soda ash solution, and use this for pH control.
The mixing plant consists of 2 x 900 litre tanks fitted with electric mixers. We will fill each tanks with 900 litres water, then add about 200 kg of dense soda ash powder to each tank. This gives a solution pH of about 11.8 when mixed. Can anyone advise how long this solution should be mixed to ensure that the powder is completely dissolved. Also, once the solution is mixed, if it is not used for some time and just sits in the tanks, can the soda ash re-crystallise and settle out in the bottom of the tank? Appreciate any advice.
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I can't advise you on the full matter, but clearly the answers will depend on the mixers, the blades, the mixer speed, the rate of addition of the ash, water temperature and maybe a few other things.
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BIGEYE wrote:

I'm no chemical engineer, but I can tell you that soda ash won't crystallize out unless the water evaporates or you dissolve it hot and allow the solution to cool.
Soda ash is properly anhydrous sodium carbonate, but the name is also sometimes used for forms with differing amounts of water of crystallization (crystal carbonate: Na2CO3.7H2O and washing soda, Na2CO3.10H2O). All forms are more soluble in hot water than cold, but if dissolved cold there's no settling.
My old CRC handbook tells me that 71 grams/liter of anhydrous Na2CO3 dissolve at 0C, and 455 at 100C. I suspect that 222 grams/liter is a bit supersaturated at room temperature. You might be interested in a chart at http://www.fmcchemicals.com/Content/CPG/Images/Caustic.pdf . Page 3 provides a cautionary warning about the temperature of solution and the formation of precipitating hepta- and decahydrate forms.
Jerry
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